This invention relates generally to fractional horsepower, single reciprocating piston hermetic refrigeration compressors of the type used in household appliances and more particularly to a discharge muffler system for such compressors.
Household refrigerators and freezers generally use relatively low horsepower compressors in the range of 1/6 to 1/3 horsepower, and commonly employ a single reciprocating piston which is driven by a two-pole motor which, with a 60 Hz power supply at a nominal speed of 3600 rpm, and therefore tends to produce noise pulses in a range where the ear is very sensitive. While such compressors utilize an electric motor fixed on a cylinder block, which is resiliently mounted within a sealed heavy sheet steel casing, there still can be a substantial amount of noise transmitted to the surrounding area and this tends to be true even though vibration absorbing exterior mounts are used between the compressor shell and the frame of the refrigerator, as well as the fact that the compressor is usually mounted at the bottom rear of the refrigerator cabinet where it is largely shielded by the refrigerator itself in a typical installation.
In compressors of this type, the interior of the hermetically sealed shell is at the relatively low pressure of the return line from the evaporator while the compressor discharge, at a much higher pressure and temperature, is conducted directly through the compressor shell by a tubing arrangement that provides sufficient flexibility to accommodate the movement of the mechanism within the shell or casing. To control the flow of refrigerant gas through the reciprocating piston compressor, the open end of the cylinder is generally covered by a rigid valve plate on which are mounted reed valves for the suction and discharge sides which communicate through ports or passages in the valve plate to suction and discharge plenums located in a cylinder head overlying the valve plate.
In order to reduce the noise inherently produced in the compression process, suitable mufflers are placed at both the suction and discharge sides of the plenums in the cylinder head. While the pressure is relatively low on the suction side, and therefore the sound is more easily muffled, the pressure on the discharge side is far greater than the suction pressure and under normal running conditions may be approximately ten times the suction pressure. Assuming that the cylinder is substantially filled through the suction valve during the suction stroke of the piston, the discharge reed valve will not open until the pressure within the cylinder exceeds that within the discharge plenum, which therefore requires a pressure increase of ten times the suction pressure before the gases can be discharged into the discharge plenum. Because the pressure must build up to the much higher level, the discharge valve does not open until, assuming a 10:1 ratio of pressures, the last ten percent of the piston stroke which may, assuming sinusoidal motion, be as low as 36 degrees of crankshaft rotation. For this reason, the pulsations at the discharge side of the compressor are shorter and sharper than those on the suction side and, this requires relatively large passages and chambers to avoid restrictions in the flow which would tend to decrease the efficiency of the compressor.
It has been recognized that to avoid excessive pressure buildup in the discharge plenum, the discharge plenum should be made as large in volume as possible and passages leading from the discharge plenum to the remainder of the muffler system should allow the gases to flow easily from the discharge plenum. On the other hand, space considerations within the shell provide some limitation on the size of the cylinder head and the discharge plenum chamber within it and the presence of large passages normally tends to decrease the muffling effect. One highly efficient discharge muffler arrangement is shown in the U.S. Pat. No. 4,401,418 of Jack F. Fritchman, granted Aug. 30, 1983, and assigned to the assignee of this application. With this arrangement, the discharge muffler system includes two large and substantially equal size muffler chambers interconnected by a tube of relatively reduced diameter compared to the other tubes handling the discharge gases. A relatively large diameter short passage conducts the gases from the discharge plenum to the first muffler chamber during the short portion of the cycle in which the gases are discharged into the discharge plenum from the pumping cylinder. The gases can then flow through the reduced diameter tube through the second muffler chamber for further expansion before being conducted through the tube to the discharge of the compressor shell. This arrangement uses a connecting tube in cooperation with the two large chambers to provide an acoustic filter which has proven quite effective in reducing noise, while providing a minimum of restriction to gas flow and therefore promoting high efficiency of the compressor.